This invention relates to digital to analog (D/A) and analog to digital (A/D) converters and more particularly to a method and apparatus for converting one digital signal to an analog signal or several digital signals to an analog signal which represents the sum of these digital signals.
Digital to analog converters presently utilize active devices such as operational amplifiers, comparators, etc. in performing the conversion functions. These active device(s) lengthen the operation time and increase the size of the D/A converter. Furthermore, most D/A converters can convert only one digital signal at a time. Those D/A converters which can convert several digital signals to an analog signal representing the sum of these digital signals need either extra time to compute the sum of the digital numbers represented by the digital signal or they utilize operational amplifiers. Either way will lengthen the D/A converter's performing time. Furthermore, many A/D converters use a D/A as a component. A D/A that is slow or large in size, means that the utilizing A/D will also be slow or large in size. The fast A/D, which does not employ a D/A, uses a large number of resistors. For example, the Type ADC0800 8-bit D/A Converter shown and described in the National Semiconductor publication entitled "Data Conversion/Acquisition Databook", 1984, National Semiconductor, requires 256 series resistors.